Implantable prostheses are commonly used in medical applications. One of the more common prosthetic structures is a tubular prosthesis which may be used as a vascular graft to replace or repair or otherwise correct a damaged or diseased blood vessel. To maximize the effectiveness of such a prosthesis, it should be designed with characteristics which closely resemble that of the natural body lumen which it is repairing or replacing.
One form of a conventional tubular prosthesis specifically used for vascular grafts includes a textile tubular structure formed by weaving, knitting, braiding or any non-woven textile technique processing synthetic fibers into a tubular configuration. Tubular textile structures have the advantage of being naturally porous, which allows desired tissue ingrowth and assimilation into the body. This porosity, which allows for ingrowth of surrounding tissue, must be balanced with fluid tightness to minimize leakage during the initial implantation stage.
Grafts are typically flexible to provide compliance within a bodily lumen or within the bodily system. Such flexibility may result from the stretching of the textile yarns forming the graft. Such stretching, however, may effect the securement of the graft to the bodily lumen, which is typically secured by the use of sutures. In other words, the graft flexibility may create undesirable stresses at the suture locations of the implanted graft.
It is also well known to form a prosthesis, especially a tubular graft, from polymers such as polytetrafluoroethylene (PTFE). A tubular graft may be formed by stretching and expanding PTFE into a structure referred to as expanded polytetrafluoroethylene (ePTFE). Tubes formed of ePTFE exhibit certain beneficial properties as compared with textile prostheses. The expanded PTFE tube has a unique structure defined by nodes interconnected by fibrils. The node and fibril structure defines micropores, which facilitate a desired degree of tissue ingrowth while remaining substantially fluid-tight. Tubes of ePTFE may be formed to be exceptionally thin and yet exhibit the requisite strength necessary to serve in the repair or replacement of a body lumen. The thinness of the ePTFE tube facilitates ease of implantation and deployment with minimal adverse impact on the body.
While exhibiting certain superior attributes, ePTFE tubes are not without certain disadvantages. Grafts formed of ePTFE tend to be relatively non-compliant as compared with textile grafts and natural vessels. Further, while exhibiting a high degree of tensile strength, ePTFE grafts are susceptible to tearing. Moreover, suture hole bleeding is a problem associated with conventional ePTFE grafts. Thus, the ePTFE grafts lack many of the advantageous properties of certain textile grafts.
It is also known to use vascular grafts in conjunction with support structures. Such support structures typically come in the form of stents, which are formed of metal or polymeric materials generally formed in a tubular structure and are used to hold a vein or artery open. Stents are well known in the art and may be self-expanding or radially expandable by balloon expansion. Examples of stent/graft configurations known in the art can be seen in U.S. Pat. Nos. 5,700,285; 5,749,880; and 5,123,917, each of which are herein incorporated by reference. It is advantageous to use stent/graft configurations because the stent provides and ensures the patency of the prosthesis, while the vascular graft provides biocompatible properties in a vessel more suitable for blood to flow.
While using a vascular graft in conjunction with support structures offers certain benefits, it is also known that support structures such as a stent can result in axial elongation and radial shrinkage of the graft material due to the stresses applied to the graft material by the support structure during the contraction and expansion of the support structure. Such stent/graft designs consist of a stent and covering. In certain situations, the covering may be attached by stitching or suturing the covering to the stent wires. In such embodiments, one of the primary failure modes of stent grafts is the loss of covering integrity at suture attachment sites. At this attachment site the textile or other material is stressed either during loading or over time by the pulsate blood flow and changing aneurysm morphology. The stresses over time cause the suture or stitching material to elongate the suture hole in the covering. Depending on the strength of the base material the holes may elongate to a point whereby the device may lose integrity.
It is apparent that conventional textile prostheses as well as ePTFE prostheses have acknowledged advantages and disadvantages. Neither of the conventional prosthetic materials exhibits fully all of the benefits desirable for use as a vascular prosthesis.
It is therefore desirable to provide an implantable material and structure, preferably in the form of a tubular vascular prosthesis, which achieves many of the above-stated benefits without the resultant disadvantages associated therewith.